1. Field of the Invention
The present invention relates to an electroacoustical transducer assembly, in general, and to the apparatus for urging a backplate into cooperative engagement with a vibratile diaphragm in such a transducer, in particular.
2. Description of the Prior Art
Capacitance-type electroacoustical transducers are well known in the prior art. In such transducers, a diaphragm having an insulative layer and an electrically conductive surface has its insulative layer in contact with a grooved, irregular, electrically conductive surface of a substantially inflexible disc or backplate. The periphery of the diaphragm is maintained in a fixed position with respect to the transducer housing and a spring force urges said backplate into tensioning engagement with said diahphragm. The insulative layer, the electrically conductive surface of said diaphragm constituting a first electrode, and the conductive surface of said backplate constituting a second electrode, form a capacitor such that when a dc bias voltage is applied across said electrodes, irregularities in said backplate surface set up localized concentrated electric fields in said insulative layer. When an ac signal is superimposed on said dc bias, the diaphragm is stressed such that oscillatory formations develop causing an acoustical wavefront to be propagated from said diaphragm. A received acoustical wavefront impinging on the diaphragm produces a variable voltage across said capacitor electrodes.
In apparatus employing a transducer of the type mentioned above to measure object distance, such as the autofocus camera sold by Polaroid Corporation under its registered trademark SX-70 Sonar One Step!, the distance to the subject to be photographed is determined by the well-known technique of measuring the round-trip time-of-flight of a burst of ultrasonic energy between the ultrasonic energy generating transducer and said subject to be photographed. This type of transducer has both transmitting and receiving modes of operation. In the transmit mode, an electronic device causes the transducer to transmit a burst of ultrasonic energy toward a subject. In the receive mode, this same transducer detects ultrasonic energy reflected from said subject impinging on said transducer's vibratile diaphragm. The elapsed time from initiation of energy transmission until receipt of an echo of said transmitted energy is a fairly accurate measure of subject distance.
In a capacitance-type ultrasonic transducer such as that described in U.S. Pat. No. 4,085,297 to PAGLIA, an electrically conductive spring member is employed to urge the backplate of a transducer into cooperative engagement with the vibratile diaphragm of said transducer. In addition to its force producing function, the spring member also forms a part of the electrical circuit or path that electrically couples the transducer to electronic circuitry external of said transducer.
If such a capcitance-type transducer is operated in an environment where it is subjected to excessive mechanical vibrations after it has transmitted an ultrasonic burst of energy toward, for example, an object whose distance is to be measured while said transducer is in its receive mode waiting for the receipt of an echo of said ultrasonic burst of energy from said object, when said excessive vibrations are present, a spurious object detect signal may be generated by the transducer if the intensity of the vibrations are sufficient to temporarily separate the electrically conductive signal carrying spring member from its associated backplate. In addition, such vibration may also causes a slight lateral movement of the spring member with respect to its associated backplate and cause a change in the amount of tensioning of the vibratile diaphragm produced by said spring member, thereby causing a change in the effective gain or amplification associated with said capacitance-type transducer by such relative spring member movement.